1. Field of the Invention
This invention relates to a method for generating a radiation image signal from a recording medium which has a radiation image recorded thereon.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during exposure to the radiation, it is possible to obtain an image having a desirable density, regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected, and converted into an electric signal to be used in the reproduction of a visible image on a recording material or a display device.
In general, in radiation image read-out apparatuses, wherein an image signal is detected from a recording medium such as an X-ray film or a stimulable phosphor sheet which has a radiation image recorded thereon, light which is emitted from the recording medium and which carries information about the radiation image is photoelectrically detected and converted into an image signal. The image signal is then sampled at sampling intervals of .DELTA.x=1/2fss, which intervals correspond to the maximum spatial frequency (maximum Nyquist spatial frequency) necessary for image information. The maximum Nyquist spatial frequency necessary for image information is herein denoted by fss. The sampled image signal is digitized. The image signal obtained in this manner includes not only the information representing the radiation image but also noise which renders the radiation image rough. Most of the noise is quantum noise which is caused by the sway of quanta of radiation irradiated to the recording medium during the recording of the radiation image and the sway of light emitted from the recording medium during the readout of the radiation image.
The image signal including the quantum noise is fed into an image processing apparatus which processes the image signal in order to minimize quantum noise. However, the image quality of a radiation image is defined by not only the level of quantum noise but also contrast and sharpness. Quantum noise is incompatible with contrast and sharpness. Therefore, when the image signal is processed in order to reduce quantum noise, contrast and sharpness deteriorate to some extent. When the image signal is processed in order to improve contrast and sharpness, quantum noise increases. Accordingly, the image quality of the radiation image compromises between quantum noise and contrast, sharpness or the like.
In order to improve image quality of a radiation image further, it is necessary to reduce the quantum noise of the original image signal. One approach to reduce the quantum noise is to increase the radiation dose to an object during the recording of a radiation image. However, for the sake of safety or other factors, the radiation dose cannot be increased to a great degree.